(1) Field of the Invention
This invention relates to a hydraulic drive system, and particularly to a hydraulic drive system suitable for a construction machine such as a wheel loader or tractor which not only runs the machine but also operates its front attachment.
(2) Description of the Related Art
Most construction machines make use of a hydraulic drive system so that large operating forces can be obtained easily from a small power source and plural working members can be driven by a single power source.
A wheel loader or tractor using such hydraulic drive systems has one hydraulic pump for driving the wheels and another for driving a bucket. A single engine connected to these hydraulic pumps gives power to the wheels and the bucket.
In other words, the hydraulic drive system of a wheel loader or tractor is provided with a single engine, with a variable displacement hydraulic pump for running the vehicle and a hydraulic pump for the bucket, both pumps being driven by the engine; and a hydraulic motor for running the vehicle and a hydraulic cylinder for the bucket, both being driven by hydraulic fluids from their corresponding hydraulic pumps. The variable displacement hydraulic pump for running the vehicle is associated with a hydraulic actuator which drives a mechanism for varying its displacement (as, for example, a swash plate). The hydraulic actuator is provided with a hydraulic fluid feeding means and a low-pressure circuit. The hydraulic fluid feeding means is adapted to feed the hydraulic actuator with a hydraulic fluid of a pressure corresponding to the degree of control (hereinafter to be called "stroke") of an accelerator lever for instructing a target revolution number of the engine, while the low-pressure circuit allows the hydraulic fluid to be discharged therethrough from the hydraulic actuator.
When the hydraulic fluid of the pressure corresponding to the stroke of the accelerator lever is fed to the hydraulic actuator by the hydraulic fluid feeding means, the hydraulic actuator is caused to move to a position corresponding to the pressure of the hydraulic fluid so that the tilting of the swash plate (namely, the displacement) is controlled correspondingly. On the other hand, the revolution number of the engine is controlled in accordance with the target revolution number instructed by the stroke of the accelerator lever, whereby the revolution numbers of the individual hydraulic pumps are also controlled. The hydraulic pump for running the vehicle is therefore controlled in both revolution number and displacement by the accelerator lever, so that the delivery rate is controlled by both revolution number and displacement. As a consequence, the revolutionary, speed of the hydraulic motor (namely, the running speed of the wheels) is controlled.
In the hydraulic fluid feeding means, the relation between the stroke of the accelerator lever and the tilting of the swash plate of the hydraulic pump is set in such a way that the tilting of the swash plate reaches the maximum at a predetermined stroke smaller than the maximum stroke. In a control range of the accelerator lever beyond the predetermined stroke, the delivery rate of the hydraulic pump can thus be increased by an increase in revolution number only.
In the above hydraulic drive system, a directional control valve is interposed between the hydraulic fluid feeding means and low-pressure circuit and the hydraulic actuator. The operating direction of the hydraulic actuator is changed over when the directional control valve is operated by an operator. The hydraulic pump for running the vehicle is a double-delivery variable displacement pump. By changing the operating direction of the hydraulic actuator, the discharging direction of the hydraulic pump is switched over, and the direction of revolution of the hydraulic motor (i.e., the running direction of the wheels), is also changed.
A power output shaft of the hydraulic motor is connected to the wheels by way of a mechanical transmission having two positions (a low speed and a high speed) and a differential gear. Owing to this arrangement, either one of the two speed levels can be obtained for the same stroke of the accelerator lever, depending on the shift position of the transmission.
Incidentally, by increasing the speed of a working member of a construction machine such as a wheel loader or a tractor, efficiency is increased. Therefore, the wheel loader or tractor is often operated at a high engine speed in order to increase the delivery rate of its hydraulic pump for the bucket. Where work is performed by a combined operation of a bucket operation and running, the running speed may hence increase too much to perform the work easily in combination with the bucket operation even when the transmission is shifted to the low speed position.
As a solution to the above inconvenience, it may be contemplated that changing the gear ratio of the transmission may reduce the running speed further. When the wheel loader or tractor is merely running without doing any work, the running speed is insufficient, and the gear ratio must be changed over frequently between high speed and low speed. The above approach therefore necessitates the operator to frequently shift the transmission, impairing the running performance. In addition, such an increased gear ratio results in greater drive torques, because of which a need arises that the strength of the axles must be enhanced. However this enhancement of the axle strength leads to an increase in cost, so that the above approach is not suitable.
As another solution, a variable restrictor (which may also be called an "inching valve") may be provided in a main line extending from a directional control valve, which is interposed between the hydraulic fluid feeding means and the low-pressure circuit, and the hydraulic actuator in the hydraulic drive system, to the hydraulic actuator. The variable restrictor is operated by an operating means (for example, a pedal) so that a portion of the hydraulic fluid to be supplied from the hydraulic fluid feeding means to the hydraulic actuator is released to limit the delivery rate of the variable displacement hydraulic pump and hence to reduce the running speed. This method, however, requires the operator to operate the pedal whenever work is done involving a combined operation of running the vehicle and operating the working member, thereby requiring the operator to perform much extra work.